1. Field of the Invention
This invention relates to novel curing agents for epoxy resins that preferably are used for the preparation of aqueous epoxy resin-hardener emulsions or dispersions. These curing agents are distinguished by the fact that the end of the processing time can be recognized by an increase in the viscosity.
2. Description of Related Art
Water-dilutable epoxy resin systems have gained increasing importance among the cold-curing "water-based paints". These two component (2C) systems have outstanding properties in comparison with one component (1C) water-based paints. These 2C systems should be designed to have the following properties: only little or no solvent content; non-flammable; only little or no odor nuisance; easily processible; low sensitivity to moist substrates; good drying and rapid complete curing; excellent adhesion to most substrates; very good intercoat adhesion; good corrosion protection on metals and easy cleaning of working equipment directly after use.
European patent EP-B 0 000 605 discloses curing agents for aqueous epoxy resin dispersions which are prepared by reacting addition products of polyalkylene polyether polyols and water-insoluble polyepoxide compounds with polyamines. Unsaturated compounds that are still capable of addition reaction can be added onto the reactive groups of the polyamines.
In European patent EP-B 0 387 418, an attempt is made to simplify the difficult synthesis of a polyether-modified resin precursor by starting from polyether glycols having amino end groups. The precursor is reacted with an excess of polyamines to produce adducts in a second stage, which is similar, if not identical, to the addition reaction disclosed in EP-B 0 000 605.
European patent EP-A 0 567 831 describes aqueous curing agents comprising: (i) an emulsifier that is prepared from a polyalkylene polyether having amino end groups, an aliphatic diglycidyl ether and a fatty amine; (ii) a co-emulsifier prepared from a polyamine, an aliphatic diglycidyl ether and a fatty amine; and (iii) a bis-amine adduct of epoxy resin and a polyamine.
The curing agents described in each of the aforementioned documents have the substantial deficiency that it is not possible to indicate the end of the processing time when these curing agents are combined with most commercially available epoxy resins, water-emulsifiable epoxy resins or also solid epoxy resin dispersions. Persons skilled in the art, however, typically are used to visually seeing a clear increase in viscosity occurring toward the end of the processing time after mixing of the two-component system, when curing conventional systems, i.e. liquid and solid resin systems which can be solvent-free or can contain solvents. Visualizing the increase in viscosity is called pot life being indicated clearly visibly. Thus, by being capable of visualizing the increase in viscosity, this avoids avoids applying a system that has already reacted.
In the case of aqueous epoxy systems, an increase in viscosity which occurs promptly before the end of the duration of use can be observed exclusively with liquid resins in combination with a few aqueous hardeners, for example .RTM.Epilink DP 660 from Akzo. It has so far not been possible at all to indicate the end of the processing time with solid epoxy resin dispersions. Because of the outstanding suitability of these dispersions for cold-curing two-component systems, such as described, for example, in the German patent DE-C 36 43 751 and in the German application DE-A 41 28 487, the lack of a pot life indication also is a deficiency here.